Graphene is a monolayer of conjugated sp2 bonded carbon atoms tightly packed into a two-dimensional (2D) hexagonal lattice. Small quantities of high quality monolayer graphene can be obtained by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG), whereas large area epitaxial graphene (EG) is grown by Si sublimation of SiC. The morphology of the EG is primarily determined by the step bunching occurring in the SiC during sublimation.
Graphene is considered to be an outstanding candidate for nanoelectronic devices due to its exceptional electronic and physical properties, including its high intrinsic carrier (electron and hole) mobility and thermal conductivity. The high carrier mobility exhibited by graphene, when combined with the integration of high-κ, i.e., highly insulating, films acting as gate dielectrics in field-effect transistors (FETs), enables operation of such devices at the very high frequencies needed for RF, low power analog communications and enables such devices to overcome the limitations of the current CMOS Si-based digital logic in new device concepts.
High-κ dielectrics such as aluminum oxide (Al2O3), hafnium oxide (HfO2), tantalum oxide (Ta2O5), and titanium oxide (TiO2), etc., are important for the realization of graphene-based top-gated electronic devices such as high electron mobility transistors (HEMTs), FETs, new logic device concepts, etc. The implementation of high-k dielectrics on graphene is expected to improve the channel mobility in such devices by screening charged impurities and reducing the high leakage currents observed in traditional silicon dioxide (SiO2) gated oxides. In addition, these graphene-based electronic devices are envisioned to manifest dimensions less than 100 nm, and use of high-κ dielectrics to help meet these scaling challenges are essential in the domain of future graphene applications.
Such high-κ gate dielectrics on graphene are envisioned to comprise very thin layers having a thickness on the order of about 2-50 nm, with minimal trapped and mobile charge which would deleteriously affect device performance. Several different approaches have been attempted to create a functionalized layer on the graphene surface for uniform, conformal dielectric deposition. However, because of the sp2 bond configuration of graphene, the surface lacks the hydroxyl sites or ALD reaction sites needed for successful deposition, and none of the current approaches have successfully provided a functionalized graphene surface suitable for conformal deposition of a high-κ dielectric.
For example, pretreatment using O3 or NO2 pretreatment such as that described in D. B. Farmer, R. G. Gordon, “Atomic Layer Deposition on Suspended Single-Walled Carbon Nanotubes via Gas-Phase Noncovalent Functionalization,” Nano Lett. 6, 699 (2006) can lead to broken bonds on the graphene sheet, or introduce charged impurities, and thus does not provide an optimal surface for dielectric deposition.
Another technique uses electron beam evaporation of metal Al and Hf followed by an oxidation step. Adam Pirkle, Robert M. Wallace, Luigi Colombo, “In situ studies of Al2O3 and HfO2 dielectrics on graphite,” Appl. Phys. Lett. Vol. 95, 133106 (2009) describe a process in which 1 nm of Al or Hf metal is evaporated directly on the exfoliated graphene surface to create a metallic seeding layer which is subsequently oxidized with dry O2 to form the first layer(s) of the dielectric to be deposited. Deposition on 200° C annealed graphene surfaces was shown to significantly affect the subsequent oxidation and film composition, indicating surface preparation greatly influences successful deposition of ALD films. The Al seeding layer also produced non-uniform clusters (<10 nm) on the graphene surface, which can limit the scalability of Al2O3. Use of an HfO2 nucleation layer resulted in covalent bonding with graphene, suggesting a degradation in electronic structure that will limit device performance.
Another approach uses a buffered dielectric seeding polymer prior to ALD deposition. Damon B. Farmer, Hsin-Ying Chiu, Yu-Ming Lin, Keith A. Jenkins, Fengnian Xia, Phaedon Avouris, “Utilization of a Buffered Dielectric to Achieve High Field-Effect Carrier Mobility in Graphene Transistors,” Nano Letters, Vol. 9, No. 12, 4474-4478 (2009) describes the usage of a 10 nm low-κ dielectric polymer spin-coated directly on the graphene surface prior to depositing HfO2 via ALD. This process seems quite successful in terms of surface coverage of the oxide layer, and does not significantly degrade carrier mobility in the graphene layer, thus allowing high field-effect mobilities in the gated structures. One disadvantage of this approach is the complexity involved, including depositing the polymer with proper spinning speeds and polymer dilution rates that will ultimately control the buffered layer thickness. Improper deposition of the polymer will introduce impurities that serve as scattering centers, thus reducing mobility. Moreover, the low-κ polymer under the gate is not always beneficial in device applications and could inhibit ultimate scaling. Such factors dictate that a high level of expertise is needed to accomplish a successful deposition. In addition, the polymer described in Farmer is not readily available, further making this process unsuitable for large-scale production.
Other techniques have also been described.
Joshua A. Robinson, Michael LaBella III, Kathleen A. Trumbull, Xiaojun Weng, Randall Cavelero, Tad Daniels, Zachary Hughes, Mathew Hollander, Mark Fanton, David Snyder, “Epitaxial Graphene Materials Integration: Effects of Dielectric Overlayers on Structural and Electronic Properties,” ACS Nano. Vol. 4, No. 5, pp. 2667-2672 (2010) discusses ALD of various high-κ dielectrics on epitaxial graphene using a nucleation layer consisting of 2-5 nm of metallic Al, Hf, Ti, or Ta, which when fully oxidized in atmosphere produces a uniform seeding film. ALD was initiated by 10 water pulses to further oxide the metallic layer. Their experiments covered a range of deposition temperatures from 80-300° C. Film coverage and uniformity was found to improve with increased temperatures, but only TiO2 resulted in a conformal and continuous film with good morphology and little effect on the underlying graphene mobility.
Yu-Ming Ling, Keith A. Jenkins, Alberto Valdes-Garcia, Joshua P. Small, Damon B. Farmer, Phaedon Avouris, “Operation of Graphene Transistors at Gigahertz Frequencies,” Nano Letters, Vol. 9, No. 1, 422-426 (2009) describes fabricating exfoliated graphene on a highly resistive Si substrate with a 300 nm SiO2 layer to form a top-gated transistor. A functionalization layer consisting of 50 ALD cycles of NO2-TMA was deposited on the graphene layer prior to deposition of a 12-nm Al2O3 layer and was used to promote a gate dielectric without pinholes. These devices showed ideal 1/f frequency dependence, characteristic of FET-like behavior, but both device conductance and field-effect mobilities were reduced after ALD deposition. This suggests that terahertz graphene devices could be realized with the proper gate dielectric deposition which preserves graphene' s high mobility.
Seyoung Kim, Junghyo Nah, Insun Jo, Davood Shahrjerdi, Luigi Colombo, Zhen Yao, Emanuel Tutuc, Sanjay K. Banerjee, “Realization of high mobility dual-gated graphene field-effect transistor with Al2O3 dielectric,” Appl. Phys. Lett., Vol. 94, 062107 (2009) describes a study showing that a dual-gated graphene FET could be achieved using exfoliated graphene and an Al2O3 top-gate dielectric deposited by ALD. A 1-2 nm metallic Al layer was deposited by e-beam evaporation was used as a nucleation layer for the 15 nm Al2O3 film. ALD growth was started with an H2O oxidizing cycle at elevated temperatures to complete the metal oxidation. While this process resulted in preservation of high mobilities, the oxide dielectric constant was low indicating improvements in oxide deposition are needed.
B. Lee, G. Mordi, T. J. Park, L. Goux, Y. J. Chabal, K. J. Cho, E. M. Vogel, M. J. Kim, L. Colombo, R. M. Wallace, J. Kim, “Atomic-Layer-Deposited Al2O3 as Gate Dielectrics for Graphene-Based Devices,” ECS Transactions, 19(5), 225-230 (2009) shows that a 10 s ozone (22 wt %) pretreatment of HOPG followed by <10 nm of Al2O3 via ALD resulted in a conformal and uniform oxide film. With normal water precursors, deposition was only achieved on step edges, while ozone-incorporated deposition allows growth on otherwise inert basal plane of graphene. In addition, Raman showed little to no defect population with this technique and C-V measurements yielded a dielectric constant of ˜9. However, there is a still large amount of leakage current in these devices, suggesting that there is non-uniform film or graphene/oxide interface imperfections.
Bongki Lee, Seong-Yong Park, Hyun-Chul Kim, KyeongJae Cho, Eric M. Vogel, Moon J. Kim, Robert M. Wallace, and Jiyoung Kim, “Conformal Al2O3 dielectric layer deposited by atomic layer deposition for graphene-based nanoelectronics.” Appl. Phys. Lett. 92, 203102 (2008) describes the use of ozone as the ALD oxygen precursor in Al2O3 deposition on HOPG. It was found that an ozone pretreatment could act as an initiation for uniform oxide deposition. The thickness shown to be conformal was ˜9.5 nm suggesting it is possible to deposit thinner dielectrics on graphene. However, the step height of HOPG is an order of magnitude smaller than epitaxial graphene which could inhibit the use of extremely thin oxides.
Xinran Wang, Scott M. Tabakman, and Hongjie Dai. “Atomic Layer Deposition of Metal Oxides on Pristine and Functionalized Graphene.” J. Am. Chem. Soc., 130 (26), (2008), pp. 8152-8153, shows that ALD of metal oxides gives no direct deposition on defect-free pristine exfoliated graphene. However, defect sites and edges are easily decorated with oxide growth. These results indicate the need for functionalization of graphene is needed in order to induce uniform surface groups as active nucleation sites for ALD. Soaking the graphene in PTCA solution for 30 min. rendered the surface necessary for ALD deposition. However, this was only studied over a few microns, and no electrical characterization was done to determine the effect of PTCA on the underlying graphene properties.
T. Shen, J. J. Gu, M. Xu, Y. Q. Wu, M. L. Bolen, M. A. Capano, L. W. Engel, and P. D. Ye, “Observation of quantum-Hall effect in gated epitaxial graphene grown on SiC (0001).” APL 95, 172105 (2009) discusses the effect of a 1 nm metallic nucleation layer prior to dielectric deposition via ALD on the electrical properties of epitaxial graphene. After metallic oxidation, 30 nm Al2O3 films were deposited at 300° C., which resulted in films that showed no significant Hall mobility degradation. These films also showed QHE and SdH oscillations again leading to the viability of graphene devices. No information on morphology was provided.
S. Banerjee, T. Hemraj-Benny, and S. S. Wong. “Covalent surface chemistry of single-walled carbon nanotubes,” Adv. Mater. 17(1) (2005) pp. 17-29, discusses the chemical functionalization of carbon nanotubes to tailor the electronic and mechanical properties for unique purposes. Several functionalizations were explored including fluorination and ozonolysis. This suggests that similar functionalization could be possible on 2D graphene sheets to yield a more reactive surface for successful ALD oxide deposition.
Thus, as can be seen from the literature, drawbacks of the prior processes include their complex and time-consuming nature, high cost, utilization of materials that are not readily available, and/or production of a coated graphene that is less electrically desirable because of damage to the graphene or the nature of the dielectric material used.